Hi summersover! I don't really know how to answer that but I can say that the current depression I'm dealing with has been around for about that length of time. My therapist says it has been really hard to shift it and she has tried me on various medications and treatments. Sometimes it is a combination of meds/therapies/lifestyle that can change things.
Do you have a doctor or therapist that you can see about this? If nothing has changed in that time, it would seem that something needs to change.
I'm sorry you are dealing with this. It is exhausting, isn't it?